Enzymatic activity of the mycoparasite Pythium nunn during interaction with host and non-host fungi

Abstract

Fine structure observations of hyphal interactions between a recently described mycoparasite, Pythium nunn, and plant pathogenic fungal species suggested that enzymatic dissolution of cell walls of host fungi was involved. The fluorescence, localized about cell wall lysis at points of interaction when stained with Calcofluor White M2R New, also suggested enzymatic activity since this stain usually binds to oligomers in regions of incomplete cell wall polymers. Again, mycoparasitic activity of P. nunn releases ¹⁴CO₂ from labelled cell walls of a plant pathogenic species of Pythium and Rhizoctonia solani. Culture filtrates of P. nunn had high cellulose and β-1,3-glucanase activity when the fungus was grown on cell walls of Pythium spp. Chitinase and β-1,3-glucanase were produced by P. nunn in cultures containing cell walls of R. solani and Sclerotium rolfsii but low activity was associated with cell walls of Fusarium oxysporum f. sp. cucumerinum. These hydrolytic enzymes were produced in dual cultures of P. nunn with several Pythium spp., Phytophthora spp., Mucor sp., Rhizopus sp., R. solani and S. rolfsii but were not detected with 10 non-host deuteromycete fungi. This difference may be due to the outer cell wall layer of mucilaginous material associated with these non-host fungi since trypsin- and or KOH-treated hyphae of non-host F. oxysporum f. sp. cucumerinum released $\text{N}$-acetyl-d-glucosamine in the presence of crude enzymatic preparations from P. nunn. This suggests that the potential host range of P. nunn is limited by components on the outer layer of fungal cell walls. Pythium nunn also produced factors inhibiting growth and/or propagule germination of a plant pathogenic Pythium sp. in vitro and in soil.